Remote optothermal sensor (ROSE) standoff detection of CWAs, explosives vapors and TICs

ABSTRACT

A system and method for standoff detection of explosives, CWAs and TICs using optical techniques. Preliminary analysis indicates detection of TNT at a distance of 0.5 km with a signal-to-noise ratio exceeding 10,000. The optical/thermal techniques apparently permit unambiguous detection of the target molecules even the presence of commonly encountered interferents. The technique, named Remote Optothermal Sensor (ROSE), has the potential for standoff detection at distances greater than one (1) kilometer.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the detection of gasses at long distance andmore particularly the detection of hazardous or chemical weapon gassesby detection of optical response.

2. Description of the Related Art

Chemical warfare agents (CWAs), explosives vapors and many toxicindustrial chemicals (TICs) absorb radiation in the medium wave infrared(MWIR) and long wave infrared (LWIR) region. This is the basis of highsensitivity, high selectivity low probability of false alarm (PFA)detection of these targets using to make MWIR and LWIR laserphotoacoustic spectroscopy (L-PAS). L-PAS is a point sensor system. Themethod and technology of using tunable IR lasers, photoacousticspectroscopy sensitivity and calculation of receiver operationalcharacteristic (ROC) curve have been described in a number of papersand/or publications by the inventors and their colleagues atPranalytica, Inc. of Santa Monica, Calif. (incorporated herein byreference).

For single-ended standoff detection applications, traditional lightdetection and ranging (LIDAR) and differential absorption LIDAR (DIAL)measurement schemes are less than satisfactory because of the smallRaman scattering signal returns (for LIDAR) and small Rayleighscattering returns (for DIAL) from molecular clouds containingexplosives vapors and CWAs, especially when the target gasses arepresent at low concentrations. On the other hand, for a CWA gas cloudcontaining explosives vapors or CWAs even at ppm concentrations, theinfrared absorption coefficients are large enough (in the range of 10⁻²to 10⁻³ m⁻¹ at 1 ppm concentration) to detect their presence, if thecloud dimensions are reasonably large (tens to hundreds of meters), aswould be the case, for example, for intentional release of a chemicalwarfare agent. Table 1 gives typical numbers for Raman scattering,Rayleigh scattering and infrared absorption cross-sections. An initialchoice of 1 ppm target may be selected from the consideration of humansensitivity to Sarin exposure of which varies from 17 ppm for lethality[1] to 170 ppb for miosis [2] for a one minute exposure. Furthermore,for a standoff detection of triacetone triperoxide (TATP), 1 ppm ofvapor pressure corresponds to a TATP temperature of −5° C. [3,4].

TABLE 1 Typical values for Raman scattering, Rayleigh scattering andinfrared absorption cross-sections (note that numbers may vary by 1-2orders of magnitude depending on the specific molecular species) ProcessCross-Section Raman scattering ~10⁻³⁰ cm² molecule⁻¹ Rayleigh scattering~10⁻²⁸ cm² molecule⁻¹ Infrared absorption ~10⁻¹⁷ cm² molecule⁻¹

Infrared absorption measurements are the basis of the L-PAS sensors thatare able to detect CWAs at ppb levels in a path length of 10 cm (pointsensor). Point detection of TATP at vapor pressures down to about 1 ppbcorresponding to a TATP temperature of −45° C. using laser photoacousticspectroscopy is known [4]. Thus, measuring absorption rather thanscattering appears to be more important for standoff detection. Ofcourse, if a double-ended system were to be acceptable, standoffdetection of tens of meters of a CWA cloud would be straightforward.However, for a single-ended detection scheme, the problem issignificantly harder and provides significant additional obstacles.

Known CWAs include: Cyclosarin (GF), Sarin (GB), Soman (GD), Tabun (GA),VX, some insecticides, Novichok agents, most arsines, cyanogen chloride,hydrogen cyanide, sulfur mustard (HD, H), nitrogen mustard (HN-1, HN-2,HN-3), Lewisite (L), Phosgene oxime (CX), chlorine, hydrogen chloride,nitrogen oxides, phosgene, tear gas, pepper spray, Agent 15 (BZ), andnon-living biological proteins such as ricin and abrin. The foregoinglist is not complete, but gives an indication of the types of chemicalsthat are used as chemical warfare agents (CWAs). Similar lists are knownfor explosives and/or explosives vapors as well as toxic industrialchemicals (TICs).

One such list for TICs is available through OSHA (the U.S. Department ofLabor's Occupational Safety & Health Administration). OSHA maintains aweb site listing TICs at the URLhttp://www.osha.gov/SLTC/emergencypreparedness/guides/chemical.htmlwhich indicates as follows:

Toxic industrial chemicals are industrial chemicals that aremanufactured, stored, transported, and used throughout the world. Toxicindustrial chemicals can be in the gas, liquid, or solid state. They canbe chemical hazards (e.g., carcinogens, reproductive hazards,corrosives, or agents that affect the lungs or blood) or physicalhazards (e.g., flammable, combustible, explosive, or reactive). Thefollowing table lists the most common TICs listed by their hazard index.

TICs listed by hazard index High Medium Low Ammonia Acetone cyanohydrinAllyl isothiocyanate (CAS# 7664-41-7) (CAS# 75-86-5) (CAS# 57-06-7)Arsine Acrolein Arsenic trichloride (CAS# 7784-42-1). (CAS# 107-02-8)(CAS# 7784-34-1) Boron trichloride Acrylonitrile Bromine (CAS#10294-34-5) (CAS# 107-13-1) (CAS# 7726-95-6) Boron trifluoride Allylalcohol Bromine chloride (CAS# 7637-07-2) (CAS# 107-18-6) (CAS#13863-41-7) Carbon disulfide Allylamine Bromine pentafluoride (CAS#75-15-0) (CAS# 107-11-9) (CAS# 7789-30-2) Chlorine Allyl chlorocarbonateBromine trifluoride (CAS# 7782-50-5) (CAS# 2937-50-0) (CAS# 7787-71-5)Diborane Boron tribromide Carbonyl fluoride (CAS# 19287-45-7) (CAS#10294-33-4) (CAS# 353-50-4) Ethylene oxide Carbon monoxide Chlorinepentafluoride (CAS# 75-21-8) (CAS# 630-08-0) (CAS# 13637-63-3) FluorineCarbonyl sulfide Chlorine trifluoride (CAS# 7782-41-4) (CAS# 463-58-1)(CAS# 7790-91-2) Formaldehyde Chloroacetone Chloroacetaldehyde (CAS#50-00-0) (CAS# 78-95-5) (CAS# 107-20-0) Hydrogen bromideChloroacetonitrile Chloroacetyl chloride (CAS# 10035-10-6) (CAS#7790-94-5) (CAS# 79-04-9) Hydrogen chloride Chlorosulfonic acidCrotonaldehyde (CAS# 7647-01-0) (CAS# 7790-94-5) (CAS# 123-73-9)Hydrogen cyanide Diketene Cyanogen chloride (CAS# 74-90-8) (CAS#674-82-8) (CAS# 506-77-4) Hydrogen fluoride 1,2-DimethylhydrazineDimethyl sulfate (CAS# 7664-39-3) (CAS# 540-73-8) (CAS# 77-78-1)Hydrogen sulfide Ethylene dibromide Diphenylmethane-4.4′- (CAS#7783-0604) (CAS# 106-93-4) diisocyanate (CAS# 101-68-8) Nitric acid,fuming Hydrogen selenide Ethyl chlroroformate (CAS# 7697-37-2) (CAS#7783-07-5) (CAS# 541-41-3) Phosgene Methanesulfonyl chloride Ethylchlorothioformate (CAS# 75-44-5) (CAS# 124-63-0) (CAS# 2941-64-2)Phosphorus trichloride Methyl bromide Ethyl phosphonothioic (CAS#7719-12-2) (CAS# 74-83-9) dichloride (CAS# 993-43-1) Sulfur dioxideMethyl chloroformate Ethyl phosphonic (CAS# 7446-09-5) (CAS# 79-22-1)dichloride (CAS# 1066-50-8) Sulfuric acid Methyl chlorosilaneEthyleneimine (CAS# 7664-93-9) (CAS# 993-00-0) (CAS# 151-56-4) Tungstenhexafluoride Methyl hydrazine Hexachlorocyclo- (CAS# 7783-82-6) (CAS#60-34-4) pentadiene (CAS# 77-47-4) Methyl isocyanate Hydrogen iodide(CAS# 624-83-9) (CAS# 10034-85-2) Methyl mercaptan Iron pentacarbonyl(CAS# 74-93-1) (CAS# 13463-40-6) Nitrogen dioxide Isobutyl chloroformate(CAS# 10102-44-0) (CAS# 543-27-1) Phosphine Isopropyl (CAS# 7803-51-2)chloroformate (CAS# 108-23-6) Phosphorus oxychloride Isopropylisocyanate (CAS# 10025-87-3) (CAS# 1795-48-8) Phosphorus pentafluoriden-Butyl chloroformate (CAS# 7647-19-0) (CAS# 592-34-7) Seleniumhexafluoride n-Butyl isocyanate (CAS# 7783-79-1) (CAS# 111-36-4) Silicontetrafluoride Nitric oxide (CAS# 7783-61-1) (CAS# 10102-43-9) Stibinen-Propyl (CAS# 7803-52-3) chloroformate (CAS# 109-61-5) Sulfur trioxideParathion (CAS# 7446-11-9) (CAS#: 56-38-2) Sulfuryl fluoridePerchloromethyl (CAS# 2699-79-8) mercaptan (CAS# 594-42-3) Telluriumhexafluoride sec-Butyl (CAS# 7783-80-4) chloroformate (CAS# 17462-58-7)n-Octyl mercaptan tert-Butyl isocyanate (CAS# 111-88-6) (CAS# 1609-86-5)Titanium tetrachloride Tetraethyl lead (CAS# 7550-45-0) (CAS# 78-00-2)Tricholoroacetyl chloride Tetraethyl (CAS# 76-02-8) pyroposphate (CAS#107-49-3) Trifluoroacetyl chloride Tetramethyl lead (CAS# 354-32-5)(CAS# 75-74-1) Toluene 2.4- diisocyanate (CAS# 584-84-9) Toluene 2.6-diisocyanate (CAS# 91-08-7)

Some of the foregoing OSHA-listed TICs may be either gasses or subjectto aerosolization.

SUMMARY OF THE INVENTION

In view of the foregoing disadvantages inherent in the known methods ofdetecting gasses (from a single point or otherwise) now present in theprior art, the present invention provides a new method of detectinghazardous, chemical warfare, explosives vapors, or other gasses bydetection of optical response wherein the same can be utilized fordetecting gasses at long distance and more particularly the detection ofhazardous or weapon gasses by detection of optical response at adistance.

The general purpose of the present invention, which will be describedsubsequently in greater detail, is to provide a new method and system bywhich gasses and constituent molecules may be analyzed at a safedistance which has many of the advantages of prior detectors and manynovel features that result in a new gas detector which is notanticipated, rendered obvious, suggested, or even implied by any of theprior art detectors, either alone or in any combination thereof.

A “stand-off” system of detection of CWAs, explosives vapors and TICclouds with high sensitivity, low PFA and high selectivity is set forthherein. The technology involves using photothermal spectroscopy at adistance rather than photoacoustic spectroscopy in a point sensor localto the sample, although the underlying principles of the two techniquesare similar. CWA detection as well as explosives vapors detection atstandoff distances of 500 m to 2 km may be possible. Prospective and/orprophetic examples are provided, below.

In one embodiment of the present invention, a system for the remotedetection of gasses uses a laser tuned to a first wavelength of interestfor a first gas. The laser is adapted for illuminating (from a distance)a cloud of gas. A heat sensor is used that is adapted for detecting heatgenerated by a gas absorbing the first wavelength. In this way, thefirst gas is detectable in a gas cloud by illuminating the gas cloudwith the first wavelength and by detecting heat generated or radiated bythe first gas with the heat sensor. Additional embodiments expand uponthis process so that several gasses can be determined in a very shortperiod of time by the serial (or otherwise) illumination of the gascloud by several wavelengths.

In another embodiment of the present invention, a method for determiningthe constituents of a gas cloud with possibly several unknown componentsis set forth. The steps of the inventive method include providing asource of illumination capable of transmitting a plurality ofwavelengths and illuminating the gas cloud with only one illuminatingwavelength at a time from the source. Determination of an opticalresponse of the gas cloud to each illuminating wavelength is maderesulting in the determination of the constituent gasses of the gascloud by resolving a mole fraction for each constituent. In this way,the constituents and their degree of presence/percentage of the gascloud are determined.

BRIEF DESCRIPTION OF THE APPENDICES

The following appendices are incorporated herein by this referencethereto.

Appendix 1 is a table showing commercially available compact telescopes

Appendix 2 is a table showing commercially available infrared imagingsystems.

Appendix 3 is a list of references corresponding to the bracketednumbers used herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of infrared absorption fingerprintsof a number CWAs in the 9-11.5 μm region (top); Positions of ¹²CO₂ and¹³CO₂ laser lines (bottom).

FIG. 2 is a schematic of the Remote Optothermal Sensor (ROSE) forstandoff detection of explosives vapors, CWAs and TICs.

FIG. 3 shows graphically an infrared absorption fingerprint of SF₆ inthe 10.5 μm region. Locations of ¹²CO₂ laser lines are also shown.

FIG. 4 is a matrix equation using the principle of extracting molefractions of target gasses from measured temperature rise of the gascolumn and laboratory data of the absorption coefficients of theconstituent gasses. K is an experimentally determined constant thatincludes camera/telescope parameters, etc. that can be determined usingthis same equation under known circumstances.

FIG. 5 graphically indicates a calculated temperature rise of theilluminated column of gas (containing the target molecule) as a functionof the absorption coefficient emphasizing the region important for verystrong absorbers such as SF₆ and TEP.

FIG. 6 is a calculated temperature rise of the illuminated column of gas(containing the target molecule) as a function of the absorptioncoefficient emphasizing the region important for strong absorbers suchas TNT and Sarin.

FIG. 7 shows atmospheric emissivity as a function of water content inpercent.

FIG. 8 shows the details of the ROSE standoff detection system.

FIG. 9 is a conceptual drawing of the laser/telescope/infrared camerafor ROSE detection of CWA/explosives vapors/TICS.

FIG. 10 is a model for calculating the heat received by the IR camerafrom a linearly distributed heat source (such as laser exposed SF₆ gas).

FIG. 11 is a schematic of the ROSE system for standoff detection at 0.5km with beam expansion optics for the laser to create a 2 cm illuminatedspot at the target.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The detailed description set forth below in connection with the appendeddrawings is intended as a description of presently-preferred embodimentsof the invention and is not intended to represent the only forms inwhich the present invention may be constructed and/or utilized. Thedescription sets forth the functions and the sequence of steps forconstructing and operating the invention in connection with theillustrated embodiments. However, it is to be understood that the sameor equivalent functions and sequences may be accomplished by differentembodiments that are also intended to be encompassed within the spiritand scope of the invention.

The present invention resides in a novel system and method for standoffdetection of explosives, CWAs and TICs using optical techniques.Preliminary analysis indicates that TNT can be detected at a distance of0.5 km with a signal-to-noise ration exceeding 10,000. Opticaltechniques may permit unambiguous detection of target molecules even inthe presence of commonly-encountered interferents. The technique, namedRemote Optothermal Sensor (ROSE), has the potential for standoffdetection at distances greater than one (1) kilometer.

Referring to the drawings, where like numerals of reference designatelike elements throughout, it will be noted that in implementing thepresent invention, a remote cloud containing CWAs, explosives with highvapor pressure such as TATP, and/or TICs, may be illuminated at specificinfrared (or other) wavelengths that are characteristic absorptionwavelengths of the targets. The resulting temperature rise of theilluminated column may be measured remotely with a sensitive infraredsensor. By measuring the resulting temperature rises for correspondingwavelengths, the presence and relative percentage of the remote cloud'sconstituents can be obtained (with a sufficient number of measurements).The laser wavelength may be tuned to select a target gas/vapor and maydiscriminate against interferents by using a multiplicity ofilluminating wavelengths as has been demonstrated with laserphotoacoustic spectroscopy (L-PAS) [5, 6].

By illuminating a remote cloud of the target gas (CWA, explosivesvapors, and/or TICs) with a tunable IR laser whose tuning range overlapswith IR absorption bands of the target, unique spectral responses can beinduced, elicited, and/or caused. These spectral responses can then bediscriminatingly detected. For illustrative purposes, focus is made onCWAs as the target gasses. FIG. 1 shows the strong absorption featuresof several CWA's and the positions of ¹²CO₂ laser and ¹³CO₂ laser linesin the lower panel. The absorbances of almost all of the CWAs lie in therange of 3-6×10⁻³ (ppm·meter)⁻¹, which translate into absorptioncoefficients in the range of 7×10⁻³ to 1.4×10⁻² (ppm·meter)⁻¹. Thus acloud of dimension 70-140 meters and a concentration of 1 ppm of thetarget gas will absorb ˜67% of the laser radiation at the peak of theabsorption feature. The next challenge is to detect this absorbedradiation remotely.

An experimental equipment schematic is shown in FIG. 2 and involvesilluminating a target gas cloud 100 with a laser beam 102 that isabsorbed by the target gas 100. The absorption of the laser radiationcauses the target gas cloud to heat up in the form of an illuminatedcolumn 104.

The temperature rise of the column 104 will cause the column 104 toradiate as a black body with emissivity that is characteristic of thecomposition of the gas cloud 100 at a temperature higher than that ofthe surrounding gas, which is generally in thermal equilibrium withambient. The IR camera/telescope combination 106 is designed to look atthe column 104 that is illuminated by the tunable laser 108 andtherefore will see the column temperature as being higher than that ofthe rest of the cloud 100. The temperature rise can be quantitativelymeasured from this remote camera 110. In the infrared spectral region,most if not all of the molecules, especially large molecules of thetarget gasses, relax after being excited by the laser radiation throughvery rapid conversion of the absorbed energy into translational energyof the molecules, i.e., a temperature rise of the gas. This is themechanism that makes photoacoustic spectroscopy work. Furthermore, ifthere is no absorption in the illuminated region, there is no heating ofthe gas cloud.

The temperature rise ΔT is proportional to the total absorbed energy(from the laser beam) by the target gas cloud,ΔT∝P _(in)(1−e ^(−αl))  (1)

where P_(in) is the laser power incident on the cloud, α is theabsorption coefficient of the target gas at the particular laserwavelength and l is the cloud dimension along the excited column. Theheated gas column will be at a temperature higher than its surroundings.As described below, a typical temperature rise, ΔT, of a few degrees C.is expected. As to be expected, use of a higher power laser wouldgenerally result in a higher temperature rise.

For the purposes of evaluating the feasibility of remote photothermaldetection, the detection of a “cloud” that has been seeded with 1 ppm ofSF₆ is described herein. The description of operations given herein areprospective and/or prophetic in nature and do not reflect actualexperiments unless otherwise indicated. Sulfur hexafluoride (SF₆) may beused as a surrogate for CWA/explosives vapors because it has a strongand well defined infrared absorption feature near 10.55 μm, much likethe absorption features of many CWAs. Moreover, SF₆ is a relativelyinert and benign gas, a feature that facilitates experimentaloptimization of the remote photothermal sensing technology withouthaving to deal with real (and extremely dangerous) CWAs in the earlystages of R&D activities. Testing with real CWAs may be carried outsubsequently at appropriate government facilities.

FIG. 3 shows the infrared “fingerprint” absorption spectrum of SF₆ near10.55 μm obtained from the PNNL database [7]. Locations of ¹²CO₂ laserlines are also shown by the indicating dots. For initial calculationsand experimentation, the 10P(16) transition of the ¹²CO₂ laser forilluminating the gas cloud is used. After the SF₆ cloud has absorbed theCO₂ laser radiation, there will be a local rise in temperature due tothermal relaxation of the molecules with its gaseous environment. Theabsorbed laser light is converted into heat. The average collisionaltime at atmospheric pressure is ˜300 ps. Intramolecular relaxation timethrough the vibrational-rotational manifold within the same electronicstate for a complex molecule is ˜μs (on the order of microseconds). So,the SF₆ molecules predominantly relax through intermolecular collisions.The time for complete relaxation of the radiatively excited molecules totranslational energy of the surrounding gas (i.e., conversion to heat)is estimated to be less than a few μs. In essence, the excited SF₆molecules lose their internal vibrational energy and heat up the gasvolume very rapidly.

Hot gasses behave like blackbody radiators with finite emissivity. Inorder to calculate the energy radiated by the SF₆ cloud fromStefan-Boltzmann law, the temperature rise of the cloud in which thecolumn of SF₆ is excited and the emissivity of the gas mixturecomprising the cloud must be known. Since SF₆ is a minor constituent ofthe gas mixture (1 ppm), the emissivity will be determined primarily bythe carbon dioxide and water content of the ambient air cloud.

With a sensitive infrared detector or camera, the detection part couldbe assured (see the calculations below) because of the strong absorptionby the target gasses, all the CWAs and vapors of explosives, whosetypical absorption coefficient, α, is 10⁻²-10⁻³ (ppm·meter)⁻¹ as seenfrom FIG. 1. Therefore a cloud of gas 10-100 meters long would absorb asmuch as 10-50% of the laser radiation at the peak of the absorptionfeature. The sensitivity calculations are also presented below.

However, it should be emphasized that the presence of the target gas ina real environment (and not in clean dry air) where more than one targetgas may be present and where interferents, which could absorb radiationof the same laser wavelength as that absorbed by the target molecule,may also be present. Thus, the measured ΔT will generally havecontributions from other gasses that absorb at the same laserwavelength. That is, the coefficient of absorption of the cloud itselfat a certain wavelength, α_(λ) _(i) is

$\begin{matrix}{{\alpha_{\lambda_{i}} = {\sum\limits_{j = 1}^{n}\;\alpha_{j}}},\lambda_{i}} & (2)\end{matrix}$

where α_(j,λ) _(i) is the absorption coefficient of the j^(th) componentof the cloud at the wavelength λ_(i).

$\begin{matrix}{{{{\Delta T}_{\lambda_{i}}} \propto {\sum\limits_{j = 1}^{n}\;\alpha_{j}}},\lambda_{i}} & (3)\end{matrix}$

This is very reminiscent of the L-PAS scheme [5,6] where thephotoacoustic signal at λ_(i) is proportional to the sum total ofabsorption caused by all the components of the gas sample in thephotoacoustic cell at the wavelength λ_(i).

In order to identify any specific target gas with selectivity and lowPFA, the column temperatures, ΔT_(λ) _(i) , at a number of wavelengthscovering the absorption features of the target gas cloud need to bemeasured. Thus, there areΔT _(λ) ₁ ,ΔT _(λ) ₂ ,ΔT _(λ) ₃ m . . . ΔT _(λ) _(n)   (4)each one of which is proportional to the sum total of absorption causedby various components at the relevant wavelength.

By knowing the laboratory values of α_(j,λ) _(i) , the matrix can beinverted (requiring the spectral coefficient matrix to be a squarematrix, thus making N=M herein) and concentrations of individual targetgasses as well as those of the interferents can be obtained. From thesedata, a receiver operation characteristic (ROC) curve can be deduced forthe target using the phenomenology developed earlier by replacing thephotoacoustic signal, S_(λ) _(i) , with the measured ΔT_(λ) _(i) atvarious wavelengths [5,6]. Since the stand off detection system hereinenjoys some similarities with L-PAS, good sensitivity with low PFAshould be obtained. As shown in the matrix equation of FIG. 4 andequation (5), below, the values of α_(j,λ) _(i) are measured andobtained from the lab while the values of ΔT_(λ) _(i) are measured inthe field. Therefore, the mole fractions of each of the components inthe gas cloud can be determined. Here K is the experimentally determinedconstant that includes the camera/telescope parameters and the overlapof the field of view (FOV) of the telescope 112 and the gas columnilluminated by the laser beam 104 that can be determined using this sameequation under known circumstances.

$\begin{matrix}{{\underset{{Spectral}\mspace{14mu}{Coefficients}}{\begin{matrix}\alpha_{\lambda_{1},1} & \alpha_{\lambda_{1},2} & \ldots & \alpha_{\lambda_{1},M} \\\alpha_{\lambda_{2},1} & \alpha_{\lambda_{2},2} & \ldots & \alpha_{\lambda_{2},M} \\⋰ & ⋰ & ⋰ & ⋰ \\\alpha_{\lambda_{N},1} & \alpha_{\lambda_{N},2} & \ldots & \alpha_{\lambda_{N},M}\end{matrix}}\underset{{Mole}\mspace{14mu}{fractions}}{\begin{matrix}X_{1} \\X_{2} \\⋰ \\X_{M}\end{matrix}}} = \underset{{Measured}\mspace{14mu}{Temperature}}{K \times {\begin{matrix}{\Delta\; T_{\lambda_{1}}} \\{\Delta\; T_{\lambda_{2}}} \\⋰ \\{\Delta\; T_{\lambda_{N}}}\end{matrix}}}} & (5)\end{matrix}$As shown in FIG. 4 and matrix equation (5), above, the mole fractions oftarget gasses are determined from the measured temperature rises of thegas column at wavelength (ΔT_(λ) _(i) ) and laboratory data of theabsorption coefficients of the constituent gasses α_(λdi i) _(,j). K,per above, is an experimentally determined constant that includescamera/telescope parameters, etc. Note should be taken that the index Mis the same for the absorption coefficients and the mole fractions. Forevery unknown mole fraction to be determined, an illumination of the gascloud at a unique wavelength is made. While the mole fractions ofnitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), and water (H₂O) may beknown, it may also be useful to treat these primary constituents of airas unknowns for the determinations to be made via equation (5).

Per above, K could be resolved under controlled circumstances by usingknown concentrations of responsive gasses in the wavelength regime ofinterest. The measured temperatures may then be divided into the matrixproduct of the spectral coefficients and the mole fractions to determineK.

Feasibility Calculations for Standoff Detection of a Target Gas Cloud

As an illustrative example, calculation may be carried out for thedetection of a cloud of SF₆ released at some distance and having anaverage concentration of 1 ppm in the atmospheric “cloud.” SF₆ has acharacteristic absorption feature at ˜10.55 μm as seen in FIG. 3, and isrelatively benign for initial demonstration purposes. The cloud of SF₆exposed to a CO₂ laser beam should be heated up spatially andtemporally. The challenge is to detect the thermal emission of SF₆ inthe cloud using an IR camera (or a sensor) placed at a distance(typically >50 meters) with high sensitivity and selectivity.

This problem may be addressed by using the following procedure (givenfor purposes of example and not for those of limitation):

(1) Calculate steady state maximum temperature rise of a highlyabsorbing target when exposed to a 1 Watt CO₂ laser beam of spot size 1cm;

(2) Calculate steady state maximum temperature rise in a SF₆ cloud thathas distributed absorption;

(3) Determine time dependent behavior of laser induced heating of gas;

(4) Accommodate choices of LWIR cameras and lenses;

(5) Image the blackbody radiated power on the IR camera;

(6) Determines detection sensitivity of IR camera and gas detectionsensitivity; and

(7) Determine response times of IR cameras—phase sensitive detection.

Calculation of the Temperature Rise

To know the temperature rise of the SF₆ cloud, the heat equation in thisparticular case needs to be analyzed. The heat equation can be writtenas:

$\begin{matrix}{{C\frac{\partial T}{\partial t}} = {{- {divJ}} + E}} & (6)\end{matrix}$where T is the temperature, C is the heat capacity, J is the flux ofheat flow and E is the laser energy absorbed per unit volume per second(source term). Here, J=−K∇T, where K is the thermal conductivity. Thus,equation (6) takes the form of a regular diffusion equation:

$\begin{matrix}{{\nabla^{2}T} = {{\frac{1}{\kappa}\frac{\partial T}{\partial t}} - \frac{E}{K}}} & (7)\end{matrix}$where the thermal diffusivity κ=K/C=K/ρc, where ρ is the density and cis the specific heat per unit mass. Solution of equation (7) holds allthe information we need both in steady state (laser not chopped) andtime dependent (chopped laser) cases.

Steady State Case

In the steady-state, equation (7) reduces to Poisson's equation:

$\begin{matrix}{{\nabla^{2}T} = {- \frac{E}{K}}} & (8)\end{matrix}$

The energy absorbed per unit volume per second isE=intensity×absorption=I×e^(−αz), where I is the Gaussian intensity ofthe laser beam I=I₀e−(r²/w²), z is the depth coordinate, r is the radialcoordinate and w is the beam spot size. Using the radial (i.e.,cylindrical) symmetry of Gaussian laser beams, equation (8) can besolved by Bessel transform. The complete solution of equation (8) hasbeen given by Lax [8]:

$\begin{matrix}{{T\left( {R,Z,W} \right)} = {B{\int_{0}^{\infty}{{J_{0}\left( {\lambda\; R} \right)}{F(\lambda)}\frac{{W\;{\mathbb{e}}^{{- \lambda}\; z}} - {\lambda\mathbb{e}}^{- {Wz}}}{W^{2} - \lambda^{2}}\ {\mathbb{d}\lambda}}}}} & (9)\end{matrix}$where the dimensionless parameters are R=r/w, Z=z/w, W=α w, B=αP/2πKF(0)and P=total incident power of the laser beam. The Gaussian functionF(R)=e^(−R) ² and F(λ) is the Bessel transform of F(R). This can beinterpreted as the “increase in temperature” due to the absorption ofthe laser beam, since one may also add a solution T=const which obeysthe differential equation and the boundary conditions [8].

The general solution for the temperature shown in equation (9) can berewritten in terms of a normalized temperature rise function N(R,Z,W)and the maximum temperature rise as:ΔT _(max)(R,Z,W)=δT _(max) N(R,Z,W)  (10)

Where the maximum temperature rise is according to Lax [8]

$\begin{matrix}{{\delta\; T_{\max}} = \frac{P}{2\sqrt{\pi}{Kw}}} & (11)\end{matrix}$

Here, the total incident power is P and we assume heating confined tothe surface layer only (i.e., W→∞). The general expression of thefunction N(R,Z,W) has been shown to be per Lax [8]:

$\begin{matrix}{{N\left( {R,Z,W} \right)} = {\frac{W}{\int_{0}^{\infty}{{F(\lambda)}\ {\mathbb{d}\lambda}}}{\int_{0}^{\infty}{{J_{0}\left( {\lambda\; R} \right)}{F(\lambda)}\frac{{W\;{\mathbb{e}}^{{- \lambda}\; Z}} - {\lambda\mathbb{e}}^{- {WZ}}}{W^{2} - \lambda^{2}}\ {\mathbb{d}\lambda}}}}} & (12)\end{matrix}$where F(λ) is the Bessel transform of F(R).

In almost all practical cases, there is finite depth of penetration andit would be advantageous to know the temperature rise along the beamaxis (i.e., R=0) and at the front surface (i.e., Z=0). So, the realistictemperature can be written on the beam axis and at the front surface tobe the maximum temperature times the reduction factor N because offinite penetration depth as:ΔT _(max)(0,0,W)=δT _(max) N(0,0,W)  (13)

where

$\begin{matrix}{{N\left( {0,0,W} \right)} = {\frac{1}{\sqrt{\pi}}{\int_{0}^{\infty}{{{\mathbb{e}}^{- \frac{\lambda^{2}}{4}}\ \left( \frac{W}{W + \lambda} \right)}{\mathbb{d}\lambda}}}}} & (14)\end{matrix}$

Equation (9) has been solved by Lax [8] in terms of Dawson andexponential integrals and the solutions for large (e.g., a metal sheet)and small values (gaseous sample like SF₆ cloud) of W (i.e., for highlyabsorbing and weakly absorbing targets) areN(0,0,W→∞)→1(metal sheet)  (15)and

$\begin{matrix}{{N\left( {0,0,\left. W\rightarrow 0 \right.} \right)} = {\frac{W}{\sqrt{\pi}}\left( {{\ln\frac{2}{W}} - \frac{\gamma}{2}} \right)\mspace{14mu}({gas})}} & (16)\end{matrix}$

where γ=0.5772 is Euler's constant.

Using equations (11), (13), (15) and (16), the steady state temperaturerise in a metal sheet and in a SF₆ cloud at the front surface along thebeam axis can be calculated.

Maximum temperature rise in strong absorber examples:

Copper (K=400 W/m-K), from equations (11) and (15): ΔT_(max)=0.07° K;and

Iron (K=80 W/m-K), from equations (11) and (15): ΔT_(max)=0.353 K

This sample calculation shows, dramatically, the role played by thermalconductivity in determining the temperature rise of the object. Thelower the conductivity, higher will be the temperature rise. Thus, a gascloud has the potential of exhibiting a higher temperature rise when thelaser radiation is absorbed by the gas.

Maximum temperature rise in a weak absorber (gas cloud) examples:

The absorption coefficient of SF₆ peak at 10.55 μm wavelength can beextracted from PNNL data base [7]. PNNL data show a decadic absorbanceof 0.04 [ppm-m]⁻¹ for SF₆ at 10.55 microns.Absorption coefficient=α=ln(10)×absorbance

α=0.092 (ppm.meter)⁻¹  (17)Gas density for 1 ppm at STP=2.68×10¹³ cm⁻³

SF₆ absorption cross section=σ=9.2×10⁻⁴/2.68×10¹³  (18)σ=3.44×10⁻¹⁷ cm⁻²/molecule

$\begin{matrix}\begin{matrix}{{n_{{SF}_{6}}\left( {1\mspace{14mu}{ppm}} \right)} = {1\mspace{14mu}{ppm}\mspace{14mu}{SF}_{6}\mspace{14mu}{density}\mspace{14mu}{at}\mspace{14mu} 300K}} \\{= {2.68 \times 10^{13} \times \left( {273/300} \right)}} \\{= {2.44 \times 10^{13}\mspace{14mu}{cm}^{- 3}}}\end{matrix} & (19)\end{matrix}$

So α (1 ppm, 300K)=σ×n=9.138×10⁻⁴ cm⁻¹ and with 1 cm beam spot size wehave W=α w=9.138×10⁻⁴ (watts per cm). The thermal conductivity of air isK=0.026 W/m-K [9]. The actual maximum temperature rise (with respect toun-illuminated area) is calculated by using equations (11), (13) and(16) to be:ΔT _(max)≈4.1 K (1 ppm SF₆, 1W CO₂ laser at 10.55 μm, 1 cm radiusbeam)  (20)

In the case of a gaseous absorber, it is instructive to parameterize thetemperature rise with respect to two critical parameters: absorptioncoefficient of the gas and the spot size (radius) of the illuminatinglaser beam on the “cloud” containing the target molecule. Results ofthese calculations, using equations (11) and (16) and thermalconductivity of air to be 0.026 W/m-K, are shown in FIGS. 5 and 6 whichemphasize different regions of interest.

Shown in FIG. 5 is the calculated temperature rise of the illuminatedcolumn of gas (containing the target molecule) as a function of theabsorption coefficient emphasizing the region important for very strongabsorbers such as SF₆ and TEP. Results for using three differentillumination spot sizes, 1 cm, 10 cm and 50 cm are shown. Positions ofabsorption coefficients for SF₆ and TEP are shown as vertical bars.

From FIG. 5, it can immediately be observed that ΔT_(max)≈4.1 K, 2.7 K,and 1.8 K for spot sizes of 1 cm, 10 cms and 50 cms, respectively forSF₆. Similar information about TEP can also be read off from the figure.

From FIG. 6, it can be seen that for TNT at a temperature of ˜122° F.,ΔT_(max)≈0.53 K, 0.4 K, and 0.32 K for illumination spot sizes of 1, 10and 50 cms, respectively. Absorption coefficient for TNT has beenderived from laser photoacoustic detection studies [11]. For 1 ppm ofSarin levels, significantly below human incapacitation, the expectedΔT_(max)≈0.41 K, 0.32 K, and 0.24 K for illumination spot sizes of 1, 10and 50 cms, respectively.

FIG. 6 shows the calculated temperature rise of the illuminated columnof gas (containing the target molecule) as a function of the absorptioncoefficient emphasizing the region important for strong absorbers suchas TNT and Sarin. Results for using three different illumination spotsizes, 1 cm, 10 cm and 50 cm are shown for target concentrations of 1ppm, which for TNT corresponds to a temperature of ˜50° C.

Time Dependent Behavior of Laser Induced Heating of a Gas Cloud

It can be seen that the heat produced (from temperature rise) in the SF₆gas cloud due to laser absorption is small but finite. In such a system,convection can be neglected as a dissipation mechanism over shortperiods of time involved in lock-in detection of the radiated heat. Thisgenerally arises from the definition of convection as given by LordRayleigh which is as follows. “When heat is fed into the system from onedirection, at small values it merely diffuses (conducts) from belowupward, without causing fluid flow. As the heat flow is increased, abovea critical value of the Rayleigh number, the system undergoes abifurcation from the stable conducting state to the convecting state,where bulk motion of the fluid due to heat begins.”

Gas is not a condensed medium, so thermal diffusion cannot be considereddue to conduction (like in a metal) as a means of dissipation of heatfrom a gas. The only way the heated gas dissipates heat is throughradiation. The emissivity of normal atmosphere (˜79% N₂, 20% O₂, 350 ppbCO₂ and ˜1% H₂O) is approximately 0.8 [10]. The largest contribution tothe emissivity of air comes from CO₂ and H₂O. The concentration of CO₂is relatively constant throughout the world and 1% water content istypical in most circumstances. However, the total emissivity is only aslowly varying function of the water content and varies from 0.6 for˜0.024% to 0.9 for 3% water (see FIG. 7).

FIG. 7 shows atmospheric emissivity as a function of water content inpercent. CO₂ concentration is assumed to be constant at 350 ppb. Dataare obtained from Staley and Jurica [10]. The solid line is shown toguide the eye.

Gagne and Chin [12] report experimental results of relaxation time of ˜2μs for CO₂ laser energy absorbed in dilute mixture of SF₆ gas in normalair. This means, if the CO₂ laser is chopped at 1 kHz, the heat producedin the gas will almost instantaneously follow the chopped excitation.This argument justifies the use of phase sensitive detection to enhancethe sensitivity in this detection scheme. Furthermore, for a 1 cmdiameter CO₂ laser beam interrogating the CWA/explosives vapor cloud,wind velocities of ≦30 km/h would not lead to appreciable degradation ofthe sensitivity of the system when using 1 kHz chopping rates.

Additionally, the relatively quick relaxation time enables a gas cloudto be interrogated very quickly with a significant, or even large,number of separate, identifying wavelengths. The resulting identifyingthermal responses can be reliably gathered/detected in a short period oftime. Some wavelengths may be repeated to confirm the thermal responseof the gas cloud to the selected wavelength of laser light. Quickidentification of harmful gasses may lead directly to fewer or mayentirely prevent injuries and deaths.

Imaging of Blackbody Radiated Power on the Sensor

The sensor response can be calculated using the following procedure:

1. Calculate the total power radiated by the blackbody (in this case,the illuminated column of gas) from the “energy flux density” or“irradiance” which is power per unit area according to theStefan-Boltzmann law. The temperature of the SF₆ cloud is just themeasure of heat produced by the laser absorption, but physically what isradiated is the heat energy per unit volume per unit. time. It isassumed that the power is radiated isotropically in 4π steradians. For aCWA cloud that is small compared with the standoff distance, this isapproximately true.

2. Calculate the fraction of the total blackbody radiated power from anelemental sphere of radius 1 cm (spot size) that is imaged onto thedetector focal plane array (FPA). The imaged area on the FPA isimportant since that will be used to calculate the rise in temperaturelocally on the FPA. For maximal collection of the radiated blackbodypower, look exactly in the direction of the laser beam, which is easyusing an almost coaxial design of ROSE system (FIGS. 8 and 9). Find thecontribution of power radiated by the entire column of gas along theviewing direction. This can be done by assuming the heated gas column tobe made up of a line of elemental spherical radiators. The length ofthis full radiating element will be approximately 1/α. The powerintegrated over this entire length, along the viewing direction, z, ofheated gas column will be imaged onto the FPA.

3. Calculate the rise in temperature knowing the fractional power beingimaged onto the imaged area on the FPA. This will yield the detectorresponse. The number of pixels illuminated by the imaged heated volumeof the gas cloud will depend on the object area, the field of view (FOV)of the telescope, the distance, and the pixel density (pitch) of theFPA. The smaller the FOV of the telescope and the higher the pixeldensity (smaller pixel pitch) of the FPA, the higher the spatialresolution of viewing will be. Ideally, the 1 cm diameter column of theheated cloud is imaged on a single pixel of the camera to get themaximum contrast between the neighboring pixels. By knowing the detectorresponse (detectivity) of one pixel (material and electronic readout)and the photon flux incident on it, the signal-to-noise ratio can becalculated accurately. This quantification of camera response becomesessential to estimate the correct PFA of the standoff detection system.The NETD in the industry refers to the noise equivalent temperaturedifference “on-the-scene” which is not a complete quantitativecharacterization of the optics-camera system like “detectivity”characterization of a detector.

A proposed schematic of a CO₂ laser irradiating a 1 cm² area of SF₆cloud at a distance of 50 m from the sensor (source-camera) system isshown in FIG. 8.

FIG. 8 shows the details of a ROSE standoff detection system. Thesource-detector system is placed at a distance of ˜50 meters from thetarget gas. For a pixel density 640×480, a viewing pixel of roughly 1cm² will be imaged on a detector having pixel dimensions of 30 μm×30 μm(specification of a typical LWIR camera).

FIG. 9 shows a conceptual implementation of the Remote OptothermalSensor (ROSE) system. For purposes of illustration and discussion, thetelescope 112 for light collection is a compact 50 cm diameter f/2catadioptric system (see Appendix 1 for the details of the telescopes).The wavelength tunable CO₂ laser 108 is mounted on the telescope chassisand the CO₂ laser beam 102 propagates collinearly and/or coaxially withthe observing axis of the telescope 112. The ability to observe theexcited cylindrical volume end on, as opposed to a small angle as shownin FIG. 2 for explanation of the principle is ideal for the presentdescription since one can visualize the imaging of the entire column ona single pixel of the camera 110 mounted at the rear of the telescope.

Power Radiated by a Heated Column of Gas

The power density of radiation emitted by a blackbody is given byStefan-Boltzmann law:J=εσT ⁴  (21)

where ε is the emissivity, σ is the Stefan-Boltzmann constant and T isthe temperature of the blackbody in degrees Kelvin. The power density ofheat radiated by the SF₆ cloud exposed to CO₂ laser radiation withrespect to its surroundings (unexposed and/or no target gas) will be:

$\begin{matrix}\begin{matrix}{J = {{ɛ\sigma}\left( {\left( {T + {\Delta\; T}} \right)^{4} - T^{4}} \right)}} \\{= {{ɛ\sigma}\left( {\left( {T^{2} + {2T\;\Delta\; T} + {\Delta\; T^{2}}} \right)^{2} - T^{4}} \right)}}\end{matrix} & (22)\end{matrix}$

Since ΔT is small compared to T, we approximate (ΔT)²→0, So we have,

$\begin{matrix}\begin{matrix}{J \approx {{ɛ\sigma}\left( {\left( {T^{2} + {2T\;\Delta\; T}} \right)^{2} - T^{4}} \right)}} \\{\approx {{ɛ\sigma}\left( {\left( {T^{4} + {4T^{3}\Delta\; T} + {4T^{2}\Delta\; T^{2}}} \right) - T^{4}} \right)}}\end{matrix} & (23)\end{matrix}$

Again using (ΔT)²→0 we have:J≈4εσT ³ ΔT  (24)

The total power radiated spherically in 4π steradians will be P=J×A,where A is the radiating area. Here the radiating area is taken as acylinder of 1 cm diameter and a length of α⁻¹ cm, where α is theabsorption coefficient. For 1 ppm SF₆ at 10.55 microns, α˜0.001 cm⁻¹,and the resulting length is approximately 10 meters.

Since the heated cylinder is viewed end-on, the total radiated poweralong the narrow cone angle subtended by the telescope FOV by the 1/αlength of the heated gas cloud needs to be calculated. Assuming acollimated laser beam, the radiating element will be a cylindrical rodof spot size radius and length 1/α. The heat radiated from the exposedcylindrical SF₆ gas column that reaches the IR camera will becontributed from its entire length. In order to use the sphericallysymmetric heat radiation model from a unit sphere of gas, the entirecylindrical gas column is broken up into a line of elemental spheres asshown in FIG. 10.

FIG. 10 shows a model for calculating the heat received by the IR camerafrom a linearly distributed heat source (laser exposed SF₆ gas).

The line of elemental spheres represents the heated gas column 104 oflength α⁻¹, the effective absorption length of the laser beam 102. Theeffective absorption length is basically the distance in which the beamintensity reduces by a factor of α⁻¹ due to the absorption by the targetgas. Each unit sphere of area A, as given above, radiates a sphericallysymmetric radiation pattern and contributes to the total thermalradiation received by the observation cone of the telescopic optics 112of IR camera 110. As the laser beam 102 is attenuated on propagation,the temperature rise of each sphere however is not the same ascalculated for the front surface (Z=0) in Equation (13) because thelaser beam intensity diminishes as the beam propagates. So, to calculatethe power contribution J×A from each sphere from Equation (24), ΔT foreach sphere along the propagation of the laser beam 102 must be known.Ideally this solution exists in using Equation (12) in Equation (10).Here, we assume an exponential decay of ΔT vs. Z (Beer's law), isassumed, i.e., we are neglecting laser beam spreading over the 1/αdistance. So it can be writtenΔT _(max)(0,Z,W)≈ΔT _(max)(0,0,W)e ^(−αZ)  (25)

where ΔT_(max)(0,0,W) has been estimated to be 4.1 K for 1 ppm SF₆ gasat 10.55 μm and Z is the propagation axis.

Considering the first unit sphere, the fractional power, P_(Fr),received by the optics is:

$\begin{matrix}{P_{F_{r}} = {\left( \frac{a_{T}}{4\pi\; D^{2}} \right)P}} & (26)\end{matrix}$

where a_(T) is the aperture area of the receiving optics (receivingoptics diameter d_(T)), D is the distance and P is the power radiated bythe first unit sphere (FIG. 10).

Now P=J×A=4εσT³ΔT×A where A=4π×10⁻⁴ m² (surface area of the elementalradiation sphere) for a 1 cm diameter laser beam. The contribution fromdifferent spheres will come from two factors, namely the distance D andexp(−αZ) (Equation 25). Assuming a collimated laser beam andsubstituting all the factors in the above equation and considering thedistance between consecutive spheres to be 2w (i.e., diameter of anelemental sphere) we have:

$\begin{matrix}{P_{F_{r}} = {\frac{a_{T}}{4\pi}4{{ɛ\sigma}T}^{3}\Delta\;{T_{\max}\left( {0,0,W} \right)}A \times {\sum\limits_{i = 1}^{\frac{1}{2w\;\alpha}}\;\frac{{\mathbb{e}}^{{- 2}w\;{\alpha{({i - 1})}}}}{\left( {D + {2{w\left( {i - 1} \right)}}} \right)^{2}}}}} & (27)\end{matrix}$

In the present case, w=1 cm, α˜0.001 cm⁻¹ for 1 ppm SF₆ at 10.55microns, and D=50 m (FIG. 3), we have:

$\begin{matrix}{P_{F_{r}} = {\frac{a_{T}}{4\pi}4{ɛ\sigma}\; T^{3}\Delta\;{T_{\max}\left( {0,0,W} \right)}A \times {\sum\limits_{i = 1}^{500}\;\frac{{\mathbb{e}}^{{- 2}\;{\alpha{({i - 1})}}}}{\left( {5000 + {2\left( {i - 1} \right)}} \right)^{2}}}}} & (28)\end{matrix}$

Evaluating

${\sum\limits_{i = 1}^{500}\;\frac{{\mathbb{e}}^{{- 2}\;{\alpha{({i - 1})}}}}{\left( {5000 + {2\left( {i - 1} \right)}} \right)^{2}}} = {1.455 \times 10^{- 5}}$

and using σ=5.67×10⁻⁸ J s⁻¹ m⁻² K⁻⁴

-   -   T=300 K,    -   ΔT_(max()0,0,W)=4.1 K    -   ε√0.8

$a_{T} = {{\frac{\pi}{4}\left( d_{T} \right)^{2}} = {\frac{\pi}{4} \times (0.5)^{2}m^{2}}}$

we have:P _(Fr)=66 μW  (29)

The value of ε is obtained from Staley and Jurica [10] and FIG. 7.

This fractional power will be imaged onto the focal plane array of theIR camera. Also note that for only one elemental sphere (i=1) at thefront (at Z=0), with a temperature rise of 4.1 K, we have P_(Fr)≈0.18μW. The difference between the total received power in Equation (29) andthis number comes from the contribution of all the elemental spheresconsidered in the summation above.

An object area 105 cm×105 cm (FIG. 8) is imaged onto an active area of20 mm×20 mm (FPA), therefore the imaged area is reduced by a factor2²/105²=3.63×10⁻⁴. For a 30 μm×30 μm pixel pitch FPA (typical for MCT,mercury cadmium telluride) we can expect to image an object of 1 cm spotsize at a 50 m distance to cover an area equal to 126 pixels on thecamera.

Detection Sensitivity of ROSE

In this section, the signal-to-noise ratio for the detection of the“heated” column of gas is calculated when either an FPA camera or asingle element detector is used.

ROSE Detection Sensitivity Using a Camera with an FPA and a Telescope

To be able to quantitatively characterize the operation of ROSE devicesthe way we have done in our L-PAS point sensors, camera performance mustbe carefully defined differently from “on-the-scene” NETD (noiseequivalent temperature difference) as is customary in the industry.Although responsivity (volts per watt) is a performance measure of aparticular IR detector, it is not useful as a standardized definitioncomparing different detector performance. On the other hand,interrelated definitions of NEP, D* & NETD involving signal-to-noiseratio offer useful figures-of-merit for comparing different FPAs.

The noise in a photon detector (not thermal) as in the present case (MCTFPA) is background fluctuation limited at cryogenic temperatures(Sterling cycle cooled FPA) and becomes generation-recombination limitedas the temperature is raised. The NETD measuring the backgroundfluctuation noise is a figure-of-merit for FPAs taking into account theoptics, array and readout electronics, not including the display [13,14, 15]. The background fluctuation noise, which limits the detectorperformance, essentially comes from the random fluctuation oftemperature due to the heat exchange between the detector and itssurroundings. Assuming only photon noise from radiating background (allother noise sources being negligible), NETD has been defined as [13, 14,15]:

$\begin{matrix}{{N\; E\; T\; D} = \frac{4F^{2}\sqrt{B}}{\sqrt{A_{\det}}\tau_{0}D*\left( \frac{\Delta\; P}{\Delta\; T} \right)_{\lambda_{1} - \lambda_{2}}}} & (30)\end{matrix}$

where

$F = \frac{f_{T}}{d_{T}}$is the f# of the telescope having a focal length f_(T) and an apertured_(T), B is the sensor bandwidth, A_(det) is the detector area, τ₀ isthe optical transmittance, ΔP/ΔT is the thermal derivative of blackbodyradiated power across the wavelength of interest λ₁-λ₂ and D* is thedetectivity.

Knowing the NETD of the MCT FPA (Santa Barbara Focalplane: SYS640/512MCT), to a catadioptric f/2 telescope (Stingray Optics: SR0322-A01),ΔP/ΔT=2.62×10⁻⁴ W/cm² K for a 8-14 microns wavelength range [13], thevalue of D* from the above equation can be estimated. Using an NETD=20mK, 1 Hz bandwidth, τ₀=0.9 and a single pixel detector size (A_(det)) of30 μm×30 μm shows:D*=1.1×10⁹ cm Hz^(1/2) W⁻¹  (31)

This D* value of a single pixel detector of a sterling cycle cooled(66K) MCT FPA is 5.5 times higher than the 0.25 mm×0.25 mm singleelement room temperature MCT detectors (D*=2×10⁸ cm Hz^(1/2) W⁻¹ formodel PVM-10.6 from Vigo Systems) presently being used. Also noteworthyis the fact that the above estimated D* of the presently availablecooled MCT FPAs is still a few orders of magnitude lower than thetheoretical performance limit of these devices [13].

To estimate an achievable “signal-to-noise” ratio in ROSE detectionscheme, the definition of the noise equivalent power (NEP) as [15] isused:

$\begin{matrix}{{N\; E\; P} = \frac{\sqrt{A_{\det}B}}{D^{*}}} & (32)\end{matrix}$

In most practical cases however, the object (laser irradiated) will befocused onto a multi-pixel image area. To be able to estimate the“signal-to-noise” ratio achievable in any image area, a generalizedexpression for the NEP is derived. This is done by substituting D* inthe expression of NEP given above. Combining equations (30) and (32) ageneralized relationship between NEP and NETD is obtained:

$\begin{matrix}{{N\; E\; P} = {\frac{A_{\det}\mspace{14mu} N\; E\; T\; D\mspace{14mu}\tau_{0}}{4F^{2}}\left( \frac{\Delta\; P}{\Delta\; T} \right)}} & (33)\end{matrix}$

For a 1 Hz bandwidth and a 30 μm square single pixel detector, we havean NEP˜2.7×10⁻¹² W per pixel.

In the above example, the 1 cm spot size (beam waist) at 50 meters isimaged roughly to a detector area 3.14×3.63×10⁻⁴ cm²=11.4×10⁻⁴ cm². Thisarea corresponds to ˜126 pixels. Thus, the power incident on each pixelis:P _(pixel)≈66/126≈0.52 μW  (34)which is just the fractional power, P_(Fr), per equation (29) divided bythe number of pixels receiving that power.

Thus the thermal radiation of 0.52 μW imaged onto a pixel gives asignal-to-noise ratio of:

$\begin{matrix}{{S\; N\; R} \approx \frac{0.52 \times 10^{- 6}}{2.7 \times 10^{- 12}} \approx {1.94 \times 10^{5}}} & (35)\end{matrix}$

ROSE Detection Sensitivity using a Single Element Detector and aTelescope

Better signal detection capability may be attained by using a properlyoptimized single element MCT (mercury cadmium telluride) detector. Acommercially available TE-cooled MCT detector (e.g., PVI-3TE-10.6 fromVigo Systems) that exhibits D*=2.5×10⁹ cm Hz^(1/2) W⁻¹ is considered. Inselecting a detector with a sensitive area of 0.5 mm×0.5 mm, a NEP(noise equivalent power or the smallest detectable power) of ˜2×10⁻¹¹ Wis used. In the above example, the 1 cm spot size at 50 meters is imagedinto an area of ˜11.4×10⁻⁴ cm², which is smaller than the 0.5 mm×0.5 mmsize MCT detector (area of 25×10⁻⁴ cm²). Thus, all of the thermalradiation of 66 μW is imaged on the detector, giving a signal-to-noiseratio (in 1 Hz bandwidth) of

$\begin{matrix}{\frac{S}{N} \approx \frac{66 \times 10^{- 6}}{2 \times 10^{- 11}} \approx {3.3 \times 10^{6}}} & (36)\end{matrix}$

The signal-noise-ratio obtained using a single element commercial gradeTE-cooled MCT detector is considerably larger than that obtained with astandard FPA camera because of the small pixel size in the camera underconsideration, which leads to the spread of the imaged area over many(˜126) pixels. Performance with specialized FPA cameras specificallysized for the ROSE application could clearly reach the capability ofsingle element commercial sensor.

Implication of the SNR Calculated for FPA Camera and a Single ElementDetector

In either case, the detection of thermal radiation from the laser powerabsorbed in the gas cloud is eminently feasible. The distance dependenceof detectability is proportional to D⁻². Thus, with the SNR of >100,000in either of the cases we can easily increase the standoff distance to 1km and retain a SNR of >300, through the use of appropriate telescopefor light/heat collection.

Proof-of-Concept for the Detection of TNT at 0.5 Km

Having derived general expressions for the standoff detection of a cloudcontaining the target gas, we can now look at a specific case ofdetection of a TNT plume from a distance. Unlike the release of a targetgas such as CWAs for which the analysis of the feasibility calculationof Section VI (above) is applicable, for the standoff detection ofexplosives, the very localized nature of the vapors release needs to betaken into account. This will limit the size of the “cloud” that isneeded for interrogation/evaluation. In the analysis below, the “cloud”will be truncated to 10 meters (rather than the α⁻¹ distance assumed fora CWA release in the atmosphere).

Again, for proof-of-concept calculations and demonstration of standoffdetection of TNT at 500 meters, SF₆ will be used as a surrogate becauseit is relatively benign and early experiments can be done without havingto go to appropriate federal facilities where needed quantities of TNTsamples can be located at 500 meter distances. By choosing appropriatevapor pressure of SF₆ the infrared absorption caused by 1 ppm of TNTwill be duplicated.

SNR Calculations (FPA Camera with 640×480 Pixels)

The SNR per pixel is given by:S/N=Power incident on FPA per pixel/NEP per pixel

The “power incident on FPA per pixel” is calculated from the fractionalpower given by Equation 27 as:

$\begin{matrix}{P_{F_{r}} = {\frac{a_{T}}{4\pi}4{ɛ\sigma}\; T^{3}\Delta\;{T_{\max}\left( {0,0,W} \right)}A \times {\sum\limits_{i = 1}^{\frac{1}{2w\;\alpha}}\;\frac{{\mathbb{e}}^{{- 2}w\;{\alpha{({i - 1})}}}}{\left( {D + {2{w\left( {i - 1} \right)}}} \right)^{2}}}}} & (27)\end{matrix}$

The received power is reduced appropriately (according to telescope FOV)from the object to the imaged area on the FPA. Knowing the area of asingle pixel, the “power incident on FPA per pixel” is equal toP_(Fr)/η, where “η” is the total number of pixels in the imaged area.The factor “η” can be written as:

$\begin{matrix}{\eta = {\left( \frac{A_{FPA}}{A_{obj}} \right) \times \left( \frac{\pi\; w^{2}}{A_{\det}} \right)}} & (37)\end{matrix}$

where, A_(FPA) is the FPA active area, A_(obj) is the target area,A_(det) is the detector area (pixel) and w is the laser spot size.

Using the aperture of telescope

$a_{T} = {\left( \frac{\pi}{4} \right)d_{T}^{2}}$and the total surface area of radiating spheres A=4πw², we have:

$\begin{matrix}{\frac{P_{F_{r}}}{\eta} = {d_{T}^{2}{ɛ\sigma}\; T^{3}A_{\det}\Delta\;{T_{\max}\left( {0,0,W} \right)}\left( \frac{A_{obj}}{A_{FPA}} \right){\sum\limits_{i = 1}^{\frac{1}{2w\;\alpha}}\;\frac{{\mathbb{e}}^{{- 2}w\;{\alpha{({i - 1})}}}}{\left( {D + {2{w\left( {i - 1} \right)}}} \right)^{2}}}}} & (38)\end{matrix}$

where the summation is over the appropriate plume depth, taken here asα⁻¹.

The generalized expression for NEP is given by Equation 33 as:

$\begin{matrix}{{NEP} = {\frac{A_{\det}{NETD}\;\tau_{0}}{4F^{2}}\left( \frac{\Delta\; P}{\Delta\; T} \right)}} & (33)\end{matrix}$

Combining Equations (27) and (33), a generalized expression is derivedfor the SNR per pixel of the FPA,

$\begin{matrix}{{S\; N\; R} = {\frac{4f_{T}^{2}{ɛ\sigma}\; T^{3}\Delta\; T_{\max}}{{NETD}\;{\tau_{0}\left( \frac{\Delta\; P}{\Delta\; T} \right)}}\left( \frac{A_{obj}}{A_{FPA}} \right){\sum\limits_{i = 1}^{\frac{1}{2w\;\alpha}}\;\frac{{\mathbb{e}}^{{- 2}w\;{\alpha{({i - 1})}}}}{\left( {D + {2{w\left( {i - 1} \right)}}} \right)^{2}}}}} & (39)\end{matrix}$

Inserting in this expression, the experimental factors including thecalculated maximum temperature rise ΔT_(max), spot size of laser beam w,distance D, absorption coefficient α, the telescope focal length f_(T),camera parameters NETD and A_(FPA), emissivity ε of the atmosphere andΔP/ΔT for the blackbody radiation, the SNR for a particular scenario canbe estimated. The summation above is made for α⁻¹ distance. However, forfinite size plumes, such as those emanating from localized explosives,the upper limit will be truncated for appropriate “plume” depth.

The SNR for a “10 meter size plume of 1 ppm TNT” surrogate at a distanceof 500 m for different spot sizes is calculated below. The parametersneeded in the SNR calculation are:

-   -   Focal length of lens f=101.6 cms (f/2.0 detection telescope)    -   Emissivity ε=0.8    -   Stefan-Boltzmann constant σ=5.67×10⁻⁸ J s⁻¹ m⁻¹ K⁻⁴    -   T=300 K    -   A_(obj)=9×10⁵ cm² for 1.2° full diagonal angle FOV and 508 mm        aperture telescope    -   A_(FPA)=4 cm², for 20 mm×20 mm FPA (640×480 pixels)=9×10⁻⁵ cm⁻¹    -   NETD=20 mK    -   τ₀=0.9

${\bullet\mspace{14mu}\left( \frac{\Delta\; P}{\Delta\; T} \right)} = {2.62 \times 10^{- 4}{Wcm}^{- 2}K^{- 1}}$

-   -   D=500 m

Putting these parameters in the above expression we have:

$\begin{matrix}{{S\; N\; R} = {25.3 \times 10^{10}\Delta\; T_{\max}{\sum\limits_{i = 1}^{n}\;\frac{{\mathbb{e}}^{{- 2}w\;{\alpha{({i - 1})}}}}{\left( {D + {2{w\left( {i - 1} \right)}}} \right)^{2}}}}} & (40)\end{matrix}$

For a plume size of 10 m, the SNR for different laser spot sizes can becalculated and the results are given in Table 2 below. Note that for afinite plume size (10 meters) the summation will be truncated fordifferent n for different spot size diameters in order to remain withinthe 10 meter size of the plume.

TABLE 2 Calculated values of SNR for detection of a 1 ppm TNT plume of10 meter dimension located at a distance of 0.5 km from ROSE.Calculations are for a 1.01 m F/2 telescope and a FPA camera with a 2 cm× 2 cm MCT array (640 × 480 pixels) Laser Spot Size ΔT_(max) TruncationTruncation Factor SNR  2 cm 0.52 K n = 250 9.38 × 10⁻⁸ 1.2 × 10⁴  4 cm0.46 K n = 125 2.78 × 10⁻⁸ 3.2 × 10³ 10 cm  0.4 K n = 50  1.88 × 10⁻⁸1.9 × 10³ 25 cm 0.36 K n = 20   7.5 × 10⁻⁷ 680

SNR Calculations (Single Element Detector)

In Section VI above, regarding imaging of blackbody radiated power onthe sensor, it was shown, that matching the pixel size to the imagedlaser beam size is the optimum detection strategy. We explore below theimprovement in the SNR over the previous case where we have used an FPAcamera as the detector. The imaged area on the detector at the telescopefocal plane is

$\begin{matrix}{a_{image} = {\pi\; w^{2} \times \left( \frac{A_{FPA}}{A_{obj}} \right)}} & (41)\end{matrix}$

For an optimum detection strategy, the detector active area to theimaged area in Equation (41) needs to be matched. The relevant NEP canthen be calculated from the D* for the MCT detector.

Substituting

$A = {{4\pi\; w^{2}\mspace{14mu}{and}\mspace{14mu} a_{T}} = {\left( \frac{\pi}{4} \right)d_{T}^{2}}}$in Equation (27), we have

$\begin{matrix}{P_{F_{r}} = {\pi\; w^{2}d_{T}^{2}{ɛ\sigma}\; T^{3}\Delta\;{T_{\max}\left( {0,0,W} \right)}{\sum\limits_{i = 1}^{n}\;\frac{{\mathbb{e}}^{{- 2}w\;{\alpha{({i - 1})}}}}{\left( {D + {2{w\left( {i - 1} \right)}}} \right)^{2}}}}} & \left( {27a} \right)\end{matrix}$

This fractional power divided by the estimated NEP of a single elementdetector gives the SNR in the case of single MCT detector sized to matchthe imaged area. Note that the NEP will change as the detector size ischanged to match the imaged area, D* remaining constant. Table 3, below,gives SNR estimates for the same parameters that were used for SNRcalculations for a FPA detector shown in Table 2.

TABLE 3 Calculated values of SNR for detection of a 1 ppm TNT plume of10 meter dimension located at a distance of 0.5 km from ROSE.Calculations are for a 1.01 m F/2 telescope and a single elementdetector (Vigo Systems model PVI-3TE-10.6 having a D* = 2.5 × 108 cmHz^(1/2)W⁻¹) sized to match the imaged area of the laser spot size onthe target plume. Laser Spot Size (cm) P_(Fr) (W) Image Size (cm²) NEP(W) SNR  2 18.4 × 10⁻⁸  5.6 × 10⁻⁵  3 × 10⁻¹²   6 × 10⁴  4 20.0 × 10⁻⁸2.24 × 10⁻⁴  6 × 10⁻¹² 3.3 × 10⁴ 10 7.36 × 10⁻⁷  1.4 × 10⁻³ 15 × 10⁻¹²4.9 × 10⁴ 25 1.64 × 10⁻⁶ 8.75 × 10⁻³ 37 × 10⁻¹² 4.4 × 10⁴

As anticipated, use of an optimized single element detector providesimproved SNR for ROSE detection of explosives. On the other hand, an FPAdetector provides a spatial image, which would be of value. Thus, for aneventual deployment of the ROSE detectors, specially fabricated FPAswill be desirable.

Proof-of-Concept Experiment

As mentioned above, the feasibility of standoff detection of TNT byusing SF₆ as the surrogate and use of the SF₆ concentration thatreplicates the infrared absorption caused by 1 ppm of TNT vapor isdemonstrated analytically. From measurements of a TNT absorption carriedout using L-PAS techniques [11] and from PNNL data base for SF₆ infraredabsorption, it is seen that ˜100 ppb of SF₆ exhibits approximately thesame infrared absorption as does 1 ppm of TNT vapor.

If a laser beam is focused to a spot size of w by a lens/mirror ofappropriate focal length. The detection of a “TNT plume” surrogate at adistance of D will be carried out using an appropriate telescope forfocusing the laser radiation into a spot size w. The required finallaser lens/mirror diameter, d, can be calculated using the expression

$\begin{matrix}{d = \frac{2\lambda\; f}{\pi\; w}} & (42)\end{matrix}$

where the f is the focal length of the final lens/mirror for focusingthe interrogating laser radiation. The confocal parameter i.e., theRayleigh range (distance over which the focused laser spot size changesby a factor of √{square root over (2)}), is given by

$\begin{matrix}{w = {{2\mspace{14mu}{cms}\mspace{14mu} 2z_{0}} = \frac{2\pi\; w^{2}}{\lambda}}} & (43)\end{matrix}$

For, w=2 cm, it is found that the final focusing lens diameter (for theproof-of-concept demonstration) to be d=15 cms. The confocal parameter(Rayleigh range) for this case is 2 z₀=237 meters. This is very longcompared to the plume dimension and therefore the laser beam spot sizeis considered as constant over the 10 meter plume dimension.

FIG. 11 is a schematic of ROSE for standoff detection at 0.5 km withbeam expansion optics for the laser 108 to create a 2 cm illuminatedspot at the target 100.

The final laser focusing lens/mirror diameter required for generating a2 cm spot size at 500 meters is only 15 cms. The experimental geometryis shown in FIG. 11. Although desirable, it is not be possible to havecollinear propagation of the laser beam 102. However, to the offsetbetween the laser axis and the detection telescope axis may be ˜33 cmwhich introduces negligible correction for end-on observation of theilluminated column of gas. Thus, from the above estimates, it is seenthat the “1 ppm TNT” surrogate 10 m plume at a distance of 500 m can bedetected using our proposed ROSE sensor with an SNR of >10⁴ using a FPAcamera 110 with a 2 cm×2 cm FPA having 640×480 pixels.

Distance Dependence of the Performance of ROSE

As a first order approximation, the excess blackbody radiation from the“heated” target gas volume received by the optical telescope varies as1/D². Thus the SNR will vary approximately as 1/D². However, a properchoice in the telescope and the FPA camera (or a single elementdetector) that permits all the collected radiation from the heatedcolumn of the target gas to be imaged on to a single pixel would makethe distance dependence somewhat slower than the 1/D². It is estimatedthat acceptable SNR (i.e., >100) can be achieved at standoff distancesas great as 2 km. Considerable more analysis is necessary to get theexact distance dependence.

Response Times of IR Camera—Phase Sensitive Detection

Appendix 2 provides data for some of the commercially available infraredimaging detectors (cameras). From this, it is noted that QWIP (quantumwell infrared photodetector) devices limit chopping speeds to severalhundred fps while MCT devices can typically run at 1600 fps. An MCT FPALWIR camera apparently is the best choice in considering the fastresponse times and allowing lock-in detection at 1 kHz for improving thesignal-to-noise ratio estimated in earlier sections.

CONCLUSION

The feasibility of standoff detection of explosives, CWAs and TICs hasbeen shown by analysis above. The problem is extremely challenging and asubstantial amount of additional analytical work may be necessary toarrive at an optimum solution for standoff detection at distances of upto more than a kilometer, which is apparently quite possible. While thepresent proposal uses a 1 W CO₂ laser as the active source fordemonstration and verification of the principle of ROSE, higher powerCO₂ lasers or high power tunable QCLs may be used in the full-fledgedimplementation of the ROSE program.

While the present invention has been described with regards toparticular embodiments, it is recognized that additional variations ofthe present invention may be devised without departing from theinventive concept. For example, if specific radiation wavelengths fromtarget gas molecules arise from predictable and reliable moleculartransitions, it may be possible to use a plurality of distinctwavelengths for target illumination if such identifying radiation can bedetected from the target gas molecules. This may further reduce the timenecessary to resolve the constituents of the target gas. Also,additional optical techniques may be used to transmit the laser beamand/or gather the resulting thermal radiation from the target gas. Suchtechniques may allow target gas detection and identification fromgreater distances.

Additionally, other chemicals may be detected by the system and methodset forth herein including the vapors and precursors for making homemadeexplosives. These will often include, among other chemicals, acetone,hexamethylenetetramine/hexamine, hydrogen peroxide, and combinationsthereof. These precursor constituents can be used for making homemadeexplosives such as TATP and HMTD.

Additionally, vapors from explosives like PETN, RDX, HMTD, ammoniumnitrate, urea nitrate, nitroglycerin, combinations thereof and otherwisemay also be advantageously pursued and possibly detected by the presentinvention.

Furthermore, in order to provide detection for a variety oflight-reactive substances, wavelength regimes of 3.5 μm to 12 μm, 0.35μm to 4 μm, 4.8 μm to 7.5 μm, 2.4 μm to 3.0 μm, and 3.4 μm to 4.0 μm maybe utilized. Corresponding light sources for these wavelength regimesinclude respectively quantum cascade lasers, interband recombinationdiode lasers, ¹²CO and ¹³CO₃, HF (hydrogen fluoride) lasers, and DF(deuterium fluoride) lasers. By exploiting optical characteristics ofsusceptible gases, and by irradiating these susceptible substances by asufficient number of wavelengths, constituents of unknownconglomerations of chemicals may be advantageously detected.

Furthermore, in providing a source of illumination for the presentinvention, sources of incoherent illumination may be used and mayinclude a blackbody light source of sufficient power to elicitsufficient response from the cloud or suspect/target gas/substance to beinterrogated optically. In using a powerful blackbody source of opticalpower generating a sufficient amount of light (in the pertinent orappropriate spectrum regime), a number of filters and/orselection/detection devices may be used. Such devices include a narrowband pass filter for the wavelength regime of interest such that theblackbody radiation is selectively chosen so that only a selectedwavelength at a time is transmitted. Additionally, a gratingspectrometer may also serve a similar purpose as may as a Fouriertransform spectrometer. A Fourier transform spectrometer may be able toprovide transmission and/or analysis of a plurality of simultaneouswavelengths in the region of interest. For the Fourier transformspectrometer, radiation detected from the gas that occurs as a result ofthe intentional irradiation thereof may be analyzed by transforming thedetected thermal signals to the appropriate wavelength domain.

APPENDIX 1 Commercially Available Compact Telescopes

TABLE 1 LWIR telescope Selection Focal Aperture Length SupplierTelescope Type (mm) (cm) FOV (°) Stingray Optics Catadioptric 508 1011.2 Model: SR0322-A01 Jenoptics-IR Lenses 130 130 6.15 DRS-infraredLenses 103 103 7.9 FUR Systems Lenses 76 76 10.55

In the model calculations in the proposal, we have used the StingrayOptics 508 mm telephoto optics (FOV˜1.2°) in combination with 640×480MCT FPA cameras (see Appendix 2).

APPENDIX 2 Commercially Available Infrared Imaging Systems

TABLE 2 LWIR Focal Plane Arrays Frames per Spectral second Pixel NETDResponse Company FPA Type (fps) Density (mK) (μm) JenopticMicrobolometer 60 640 × 480 90 7.5-14 FLIR Microbolometer 60 640 × 48090  7.5-13.5 Systems QWIPs 640 × 512 35    8-9.2 DRS IR Microbolometer60 640 × 480 90 7.5-14 SBFPMCT >1K 640 × 480 20 8-12 (Model: SYS640/512MCT) SEIR QWIPs >1K >1K 25 10-15% of MCT >1K 1K × 1K 18 peak   8-12

Comparison of the Above FPAs:

Microbolometers are broad band but slow (low fps) with relatively highNETD

QWIPs are small bandwidth devices. Low quantum efficiency (˜10%) and sorequires more integration time (˜2 ms per pixel), typically run below200 fps. Relatively low NETD.

MCTs are broadband, very high efficiency requiring fast integrationtimes (˜100 μs), can be run at 1600 fps/frames per second (a window of256×256 pixels can be run at 1000 fps), very low NETD.

QWIPs and MCTs are good choices in our case since microbolometers haverelatively high NETD and are slow. MCT FPA is the better choice in ourcase considering their broadband operation, lock-in detectionpossibility (very fast fps) and lowest NETD (15-20 mK).

Lockheed-Martin Santa Barbara Focal Plane (www.sbfp.com) and SE-IRcorporation (www.seir.com), both in Santa Barbara, integrate customerprovided FPAs (QWIPs and MCTs) and optics with closed cycle Sterlingcycle coolers and provides the drive electronics and software to makethe final LWIR camera.

REFERENCES

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1. A system for the remote detection of gasses in a gas cloud,comprising: a laser system tuned to a first wavelength of interest for afirst gas, said laser system adapted for illuminating from a distancethe gas cloud, said laser system including a tunable laser systemtunable to a plurality of wavelengths characteristic of absorptionwavelengths of said first gas in a range of approximately 9 μm to 11.5μm; said laser system tunable to a sufficient plurality of wavelengthsto discriminate target gasses against interferents, said laser systemilluminating the gas cloud with said plurality of wavelengths and withseparate illuminations for each wavelength; a heat sensor adapted fordetecting heat generated by the gas cloud when the gas cloud absorbssaid first wavelength such that said first gas is at least partiallydetectable in the gas cloud by illuminating the gas cloud with saidfirst wavelength and by detecting heat generated by at least said firstgas with said heat sensor, said heat sensor capable of sensing transientmolecular relaxation of a laser-stimulated portion of the gas cloud,said transient molecular relaxation occurring on the order ofmicroseconds, said heat sensor capable of sensing a higher temperaturein a portion of the gas cloud illuminated by said laser; and said heatsensor capable of sensing transient molecular relaxation of alaser-stimulated portion of a gas cloud for each of said separateilluminations, said separate illuminations selected from the groupconsisting of illuminations separated by intervals of time,illuminations sequentially transmitted in a continuous manner, andcombinations thereof.
 2. A system for the remote detection of gasses ina gas cloud as set forth in claim 1, wherein said laser system furthercomprises: a ¹²CO₂ laser and ¹³CO₂ laser.
 3. A system for the remotedetection of gasses, comprising: a laser tuned to a first wavelength ofinterest for a first gas, said laser adapted for illuminating from adistance a cloud of gas; and a heat sensor adapted for detecting heatgenerated by a gas absorbing said first wavelength; whereby said firstgas is detectable in a gas cloud by illuminating said gas cloud withsaid first wavelength and by detecting heat generated by said first gaswith said heat sensor.
 4. A system for the remote detection of gasses asset forth in claim 3, further comprising: said laser including a tunablelaser tunable to a plurality of wavelengths.
 5. A system for the remotedetection of gasses as set forth in claim 4, wherein said tunable laserfurther comprises: a tunable laser tunable to wavelengths characteristicof absorption wavelengths of said first gas.
 6. A system for the remotedetection of gasses as set forth in claim 5, wherein said tunable laserfurther comprises: said tunable laser tunable to wavelengths in a rangeof approximately 9 μm to 11.5 μm.
 7. A system for the remote detectionof gasses as set forth in claim 6, wherein said tunable laser furthercomprises: a ¹²CO₂ laser and ¹³CO₂ laser.
 8. A system for the remotedetection of gasses as set forth in claim 5, wherein said tunable laserfurther comprises: said tunable laser tunable to wavelengths in a rangeof approximately 3.5 μm to 12 μm.
 9. A system for the remote detectionof gasses as set forth in claim 8, wherein said tunable laser furthercomprises: a quantum cascade laser.
 10. A system for the remotedetection of gasses as set forth in claim 5, wherein said tunable laserfurther comprises: said tunable laser tunable to wavelengths in a rangeof approximately 0.35 μm to 4 μm.
 11. A system for the remote detectionof gasses as set forth in claim 10, wherein said tunable laser furthercomprises: a interband recombination diode laser.
 12. A system for theremote detection of gasses as set forth in claim 5, wherein said tunablelaser further comprises: said tunable laser tunable to wavelengths in arange of approximately 4.8 μm to 7.5 μm.
 13. A system for the remotedetection of gasses as set forth in claim 12, wherein said tunable laserfurther comprises: a ¹²CO and a ¹³CO laser.
 14. A system for the remotedetection of gasses as set forth in claim 5, wherein said tunable laserfurther comprises: said tunable laser tunable to wavelengths in a rangeof approximately 2.4 μm to 3.0 μm.
 15. A system for the remote detectionof gasses as set forth in claim 14, wherein said tunable laser furthercomprises: a HF laser.
 16. A system for the remote detection of gassesas set forth in claim 5, wherein said tunable laser further comprises:said tunable laser tunable to wavelengths in a range of approximately3.4 μm to 4.0 μm.
 17. A system for the remote detection of gasses as setforth in claim 16, wherein said tunable laser further comprises: a DFlaser.
 18. A system for the remote detection of gasses as set forth inclaim 3, wherein said heat sensor further comprises: a heat sensorcapable of sensing transient molecular relaxation of a laser-stimulatedportion of a gas cloud.
 19. A system for the remote detection of gassesas set forth in claim 18, wherein said heat sensor further comprises: aheat sensor capable of sensing transient molecular relaxation occurringon the order of microseconds.
 20. A system for the remote detection ofgasses as set forth in claim 3, wherein said heat sensor furthercomprises: a heat sensor capable of sensing a higher temperature in aportion of said gas cloud illuminated by said laser.
 21. A system forthe remote detection of gasses as set forth in claim 3, furthercomprising: said laser tunable to a sufficient plurality of wavelengthsto discriminate target gasses against interferents, said laserilluminating said gas cloud with said plurality of wavelengths and withseparate illuminations for each wavelength; and said heat sensor capableof sensing transient molecular relaxation of a laser-stimulated portionof a gas cloud for each of said separate illuminations.
 22. A system forthe remote detection of gasses as set forth in claim 21, furthercomprising: said separate illuminations selected from the groupconsisting of illuminations separated by intervals of time,illuminations sequentially transmitted in a continuous manner, andcombinations thereof.